Apple's new Safari 5 browser is the fastest browser on both Windows and Mac OS X, JavaScript benchmark tests show.

According to tests run by Computerworld, Safari 5, which Apple launched late Monday, renders JavaScript 27% faster than the nearest competitor on the Mac, and 11% faster than the second-place browser on Windows.

Safari dethrowned the speed kings from Google and Opera Software to take the top spot in the time trials.

On Monday, Apple touted Safari 5's increased speed, claiming that the browser's Nitro JavaScript engine is "up to 30% faster than Safari 4," and could also beat both Google's Chrome and Mozilla's Firefox.

Computerworld ran the SunSpider JavaScript benchmark suite in Windows XP Professional Service Pack 3 (SP3) and on Mac OS X 10.6.3 three times for each browser, then averaged the scores to arrive at the final rankings.

On the Mac, the results rated Safari 5 as 34.8% faster than the previous version of Apple's browser, Safari 4.0.5. Safari 5 beat No. 2 Opera 10.53 by 26.6% and trumped the relatively new Chrome 5's JavaScript speed by 28.9%.

Safari 5 proved more than three times faster than Firefox 3.6.4, the almost-ready Mozilla browser that's been stalled since June 1; Mozilla is trying to quash a final bug or two before shipping that edition.

On Windows, the gap between Safari 5 and its rivals was narrower. There, Safari 5 proved to be 10.6% faster than Chrome 5 and 15.6% faster than Opera 10.53, and rendered JavaScript about 2.5 times faster than Firefox 3.6.4.

Microsoft's historically sluggish Internet Explorer 8 (IE8) took 12 times longer to run the SunSpider benchmarks than Safari 5.

Apple's claim that Safari is "the world's fastest web browser" may be true today, but previous claims haven't stood up to testing. Opera, for example, grabbed the top spot in February, surging past Chrome and Safari, the former No. 1 and No. 2 browsers on Windows, and didn't relinquish it until this month.

The rankings are open to debate. JavaScript performance results can vary significantly, depending on the hardware used to test and the benchmark suite used. Several other technology sites and blogs, for example, have said that their benchmarks show Safari 5 lagging behind Chrome on Windows. Some have gone as far as to call Apple's speed claim "a flat out lie."

Although Safari may own the speed prize -- by Computerworld's testing -- it not the world's most popular browser. According to the newest data from Web metrics company Net Applications, Safari accounted for just 4.8% of all the browsers used in May, a far cry from IE's 59.7%, Firefox's 24.4% or even Chrome's 7.1%. Of the five major browsers on Windows, only Opera, with just 2.4% of the usage market, trails Safari.

Safari 5 is the fastest browser on Windows as well as Mac. Smaller results better. IE8 not shown.